Vehicle evaporative emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a fuel vapor canister containing a suitable adsorbent, and then purge the stored vapors during a subsequent engine operation. The stored vapors may be routed to an engine intake for combustion, further improving fuel economy.
In a canister purge operation, a canister purge valve coupled between the engine intake and the fuel canister is opened, allowing for intake manifold vacuum to be applied to the fuel canister. On a boosted engine, that vacuum draw may be supplied via an ejector during boosted operation. For particular hybrid vehicles, that vacuum draw may be provided via a canister purge pump positioned between the canister and the canister purge valve, for example. Simultaneously, a canister vent valve coupled between the fuel canister and atmosphere is opened, allowing for fresh air to enter the canister. Further, in some examples a vapor blocking valve coupled between the fuel tank and the fuel canister is closed to prevent the flow of fuel vapors from the fuel tank to the engine. This configuration facilitates desorption of stored fuel vapors from the adsorbent material in the canister, regenerating the adsorbent material for further fuel vapor adsorption.
A flow map stored at a controller of a vehicle may be used to command an appropriate duty cycle for the canister purge valve when purging of the canister is requested. More specifically, a particular flow value may be commanded in response to a request to purge the canister, and a 3D flow map stored at the controller may be queried to determine an appropriate duty cycle for the canister purge valve as a function of engine manifold vacuum. Such a flow value may be chosen so as to avoid potentially stalling the engine due to a rich amount of fuel vapors emanating from the canister (and fuel tank in some examples), and may be further based on a number of other relevant engine operating parameters. Furthermore, it is desirable to learn canister loading state via feedback from exhaust gas oxygen sensors during purging operations, and in order to accurately learn such a loading state, it is imperative that the commanded flow is accurate. If the canister purge valve has become degraded to any extent (or replaced), such a commanded flow value may not be accurate or representative, which may thus impair accurate learning of canister loading state. An inability to accurately assess canister loading state may adversely impact engine operation.
Such flow maps for canister purge valves may be generated via a technician offline, and are not updated via any onboard diagnostic protocol. More specifically, at a bench, a flow meter may be used via a technician to assess flow values for a canister purge valve at various vacuum levels to generate a flow map for use when the vehicle is in operation. However, as discussed above, canister purge valves may become degraded or may be in some examples replaced (with a valve having different flow characteristics), thus rendering the use of a particular flow map stored at the controller non-ideal. However, evaporative emissions systems for vehicles that include fuel vapor storage canisters and canister purge valves are not equipped with flow meters or onboard strategy to update canister purge valve flow maps as a function of an operational state of the canister purge valve (e.g. unable to properly seal when commanded closed, etc.). An inability to update such flow maps as a function of canister purge valve operational state may impact engine operation as discussed. Thus, an onboard diagnostic strategy to periodically, or in response to an indication of canister purge valve degradation, update such flow maps to more accurately reflect flow characteristics of the canister purge valve, is lacking.
The inventors herein have recognized the above-mentioned issues, and have developed systems and methods to address them. In one example, a method comprises controlling a duty cycle of a purge valve configured to regulate a purge flow from a fuel vapor storage canister to an intake of an engine during a canister purging event based on a degradation factor obtained by comparison of durations at which a predetermined pressure is reached in an evaporative emissions system at multiple purge valve activation levels. In this way, the purge valve may be controlled to avoid engine hesitation, stall, or other undesired engine operating conditions, during conditions where the canister is purged of fuel vapors to engine intake.
In one example, the evaporative emissions system is sealed for obtaining the durations at which the predetermined pressure is reached, the predetermined pressure comprises a negative pressure with respect to atmospheric pressure, and a fuel system is sealed from the evaporative emissions system for obtaining the durations at which the predetermined pressure is reached.
As another example, the comparison of the durations at which the predetermined pressure is reached at multiple purge valve activation levels includes both conditions for which there is an absence of evaporative emissions system degradation and conditions for which there is a presence of evaporative emissions system degradation.
As another example, such a method further comprises operating a pump positioned downstream of the fuel vapor storage canister to communicate a predetermined vacuum to the evaporative emissions system for obtaining the durations at which the predetermined pressure is reached. In one example the pump comprises the engine, and in another example the pump comprises a purge pump positioned in a purge line between the purge valve and the fuel vapor storage canister.
As yet another example, the predetermined pressure that is reached is monitored via a pressure sensor positioned in the evaporative emissions system configured to indicate pressure across a reference orifice in the evaporative emissions system. The multiple purge valve activation levels may include two or more different duty cycles of the purge valve. Furthermore, the degradation factor may be used to adjust a flow map for controlling the duty cycle of the purge valve during the canister purging event.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.